Size-Dependent Strength and Plasticity in Metallic Nanocrystalline-Amorphous Composites

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Materials that are strong and ductile are always favored and highly demanded by industry. However strength and ductility are usually anti-correlated in materials and are difficult to achieve simultaneously. While monolithic nanocrystalline metals and amorphous metallic glasses both exhibit higher strength as compared to conventional metals and alloys, their brittle natures severely limit their application as engineering materials. Some prior studies suggest that introducing a small portion of amorphous phase in nanocrystalline materials or vice versa can significantly improve the ductility of these materials without sacrificing the strength. For instance, exceptional tensile ductility up to ~13.8% elongation has been reported in crystalline Cu/CuZr glass nanolaminates as compared to < 2% in conventional nanocrystalline Cu. It has also been observed that introduction of ~7% volume fraction of nanocrystals can increase the compressive ductility of amorphous CuZr bulk metallic glass to > 5% from nearly zero. Although such investigations were based on trial-and-error and a deep understanding of the deformation mechanisms is still lacking, it opens the door to design materials and novel structures with desired strength and ductility at the same time. The applicant proposes to use mainly atomistic simulation and subsequent in-situ deformation experiments to study the strength and plasticity and their dependence on microstructures of crystalline/amorphous composites. The proposed study will significantly advance current understanding of the microplasticity in amorphous metallic glass and crystalline/amorphous composites in confined volumes. Ultimately, the research will contribute to the design of novel structures and materials with optimized combination of strength and ductility that can be used for various industrial applications, for example in microelectromechanical systems and nanoelectro-mechanical systems.